1. Field of the Invention
The present invention relates to communication equipment.
2. Description of the Related Art
Different waveforms may be used to transmit digital data streams. Two such waveforms (non-return-to-zero (NRZ) and return-to-zero (RZ)) of particular relevance to the present invention can be characterized as follows. In an NRZ data stream, two consecutive bits of logical “ones” represented by a high level in the carrier signal are transmitted without the carrier signal falling to a low level between the bits. In contrast, in an RZ data stream, the carrier signal returns to the low level between bits. For example, an NRZ signal representing a relatively long string of logical “ones” appears to have a DC nature, while an RZ signal representing the same string appears as a sequence of pulses.
Most electronic systems transfer data using NRZ. Similarly, in fiber optic communication systems, on/off modulation of laser light using NRZ is the most commonly used method of data transmission. However, substituting NRZ with RZ is being increasingly considered in modern optical network designs, since the latter can provide certain advantages. For example, in long distance transmission, an RZ optical signal is less susceptible to non-linearities and polarization mode dispersion than a corresponding NRZ optical signal. Therefore, converting NRZ electronic data streams into optical RZ signals for transmission over optical networks is an emerging need.
FIG. 1 shows a typical prior art system 100 for converting an NRZ electronic data stream 102 into an RZ optical signal 104. System 100 comprises an optional multiplexer (MUX) 120 for combining two or more tributary NRZ data signals 118 into data stream 102 and deriving a reference clock signal 112. Signal 112 may be a sine wave at a reference clock frequency. System 100 further comprises a laser 106 that generates a continuous wave (CW) beam of light. This beam is fed into an optical fiber and transmitted to a first electro-optic (E/O) modulator 108. Modulator 108 is configured to generate an optical pulse train using a modulator driver 114 receiving input signal 112. The output of modulator 108 is then an optical pulse train at that frequency. The output of modulator 108 is fed into a second E/O modulator 110, which can be similar to modulator 108. Modulator 110 is configured to modulate the optical pulse train using a second modulator driver 116 receiving data stream 102. The output of modulator 110 is RZ optical signal 104.
One problem with system 100 is that it requires two E/O modulators (108 and 110) and two modulator drivers (114 and 116) adding to the cost of the system. Another problem with system 100 is that it requires synchronizing an optical pulse train generated by modulator 108 and electronic data stream 102. Such synchronization is difficult to maintain due to often occurring and, in general, poorly controllable phase drifts in E/O modulators and/or associated electronics.